1. Field of the Invention
The present invention relates to a contrast echo image analysis method employed for detection or treatment of ischemic heart diseases or similar diseases, in which a contrast echo image of the myocardium and cardiac chambers—which image is obtained by use of ultrasound diagnostic equipment after injection of a microbubble ultrasound contrast agent into blood vessel—is analyzed so as to visually and quantitatively determine the volume of blood flowing through the myocardium on the basis of intensity values of the myocardium region and the cardiac chamber regions of the image.
2. Background Art
X-ray diagnostic equipment, X-ray CT equipment, MRI equipment, or nuclear medical diagnostic equipment enables diagnosis of interior tissues of a human body without surgical operation, but such equipment involves the following problems: a subject is exposed to X-rays or nuclear radiation, and examination requires a long period of time. Meanwhile, catheterization is an invasive examination technique in which a catheter is inserted into a blood vessel. In contrast, ultrasound diagnostic equipment, which employs ultrasound waves, causes virtually no adverse effects on body tissues of a subject—which would otherwise be incurred through exposure of the subject to X-rays, nuclear radiation, or magnetic field during use of the aforementioned equipment; i.e., ultrasound diagnostic equipment is non-invasive diagnostic equipment. Ultrasound diagnostic equipment attains real-time image display, involves low risk even when repeatedly used within a short period of time, and enables examinations to be completed within a short period of time. Furthermore, ultrasound diagnostic equipment is small in size and is inexpensive, and thus can be used in any hospital. In addition, such ultrasound diagnostic equipment allows a medical doctor to readily and thoroughly examine a region of interest (ROI) while directly operating the equipment.
When ultrasound diagnostic equipment is used, an ultrasound probe is placed on the body surface in order to receive reflection waves of ultrasound waves emitted from a transducer, and tomographic images of body tissues are obtained on the basis of the received reflection waves. Therefore, for example, the state of the heart, the abdomen, or the mammary gland, or movement of a fetus within the uterus can be observed in real time. Meanwhile, through use of the power Doppler method, imaging of the blood flow can be performed.
The blood that has circulated throughout the body flows through veins to the right atrium and the right ventricle, and then flows from the right ventricle through arteries to the lungs. The blood is oxygenated in the lungs. The blood that has been oxygenated in the lungs passes through the left atrium and the left ventricle, and flows through arteries to the entire body. During this circulation, a portion of the thus-oxygenated blood is fed to coronary arteries, which are branching from the aorta. The coronary arteries, which cover the myocardium while forming a web-like network, supply oxygen and nutrients, which energize the heart, to the myocardium. When the coronary arteries are occluded or stenosed due to formation of thrombi or occurrence of coronary artery disorder, the amount of the blood supplied to the heart is reduced, causing adverse effects on action of the heart. When the coronary arteries are occluded, and necrosis of myocardial cells occurs, the contraction force of the myocardial cells is lost. This symptom is called “myocardial infarction.” When the amount of the blood supplied to the myocardium is reduced as a result of narrowing of the coronary arteries, although occlusion of the arteries does not occur, the heart is adversely affected. This symptom is called “angina pectoris.” Myocardial infarction and angina pectoris are collectively called “ischemic heart diseases.”
In the case where the heart is examined by use of ultrasound diagnostic equipment, a contrast agent is injected via a vein in order to evaluate blood perfusion, because the intravenous injection of a contrast agent is less invasive. In diagnosis by use of a contrast agent, change with time in spatial distribution of the contrast agent in a site to be diagnosed is observed on the basis of an increase in an intensity-enhanced area and an increase in intensity. In addition, there is obtained the transit time between the injection of the contrast agent and the arrival thereof to a region of interest (ROI), as well as the time intensity curve (TIC), which represents change with time in intensity of an echo image of the ROI captured by use of a contrast agent.
Ultrasound waves (echo signals) reflected from an organ of a living organism tend not to exhibit non-linear behaviors. However, echo signals obtained by use of an ultrasound contrast agent predominantly containing microbubbles include non-fundamental wave components attributable to non-linear behaviors. Therefore, when only the non-fundamental wave components are separated from the echo signals and detected, an organ of a living organism and a cavity (e.g., the interior of a blood vessel or the cardiac chambers of the heart) can be observed at a high contrast ratio in an image generated by a contrast agent.
The aforementioned contrast echo method is disclosed in, for example, Japanese Patent Application Laid-Open (kokai) No. 11-155858 (title of the invention: “Ultrasound Diagnostic Equipment and Ultrasound Diagnostic Method”) and Japanese Patent Application Laid-Open (kokai) No. 2001-178722 (title of the invention: “Ultrasound Diagnostic Equipment and Ultrasound Diagnostic Method”). The former patent document discloses a method for obtaining a more effective contrast echo image, in which the transmission sound pressure of ultrasound pulses is optimized in order to increase the intensity of an image obtained by use of a contrast agent. The latter patent document discloses a method and equipment for reducing labor or burden of an operator when carrying out a contrast echo method employing an ultrasound contrast agent predominantly containing microbubbles. This patent document proposes a method for informing a diagnostician of vanishment of microbubbles in the form of sound information, by use of a speaker.
In the case where the heart is examined by use of ultrasound diagnostic equipment and in accordance with the contrast echo method employing an ultrasound contrast agent predominantly containing microbubbles, a high level of skill is required for diagnosing ischemic heart diseases (e.g., myocardial infarction and angina pectoris) on the basis of the resultant contrast echo images. The reasons for this are described below.
When a microbubble ultrasound contrast agent is injected into blood vessels, and an ultrasound probe is placed on the body surface to thereby apply to a region of interest (ROI) ultrasound waves transmitted from the probe, the ultrasound waves are reflected by microbubbles contained in the contrast agent. Ultrasound waves of high energy are reflected from a site where blood containing the contrast agent is present. When received reflective waves are converted into intensity and an image is displayed on the basis of the intensity, a portion of the image corresponding to such a site is displayed with a higher intensity; i.e., as a bright region. A site to which the contrast-agent-containing blood is not supplied in a sufficient quantity is displayed with a lower intensity; i.e., as a dark region. Therefore, whether or not the blood is sufficiently supplied to the myocardium can be determined through comparison of the intensity of the myocardium at different locations.
Ultrasound waves transmitted from the probe attenuate with the distance between the body surface and a relevant site of the body; i.e., the depth of the site. Further, since the probe focuses ultrasound waves on a relevant site, the energy level at that site becomes higher than that of other sites. Moreover, in the case where a color image of the heart is obtained by means of the power Doppler method or the B-mode method, the color of image regions corresponding to the cardiac chambers bleeds into an image region corresponding to the myocardium. Therefore, difficulty is encountered in correctly determining the state of perfusion of blood on the basis of contrast echo images of the myocardium and the cardiac chambers.
As described above, in the case of a conventional contrast echo image, intensity varies from site to site because of depthwise attenuation of ultrasound waves and energy maximization at a site where ultrasound waves are focused. However, in the conventional image processing technique, intensity measurement is performed without correction of variation in intensity, which variation occurs due to the difference in acoustic field or focusing of ultrasound waves. Therefore, in the conventional technique, difficulty is encountered in determining whether the difference in intensity between different sites in the cardiac chambers or the myocardium is caused by the difference in acoustic field or by the difference in perfusion.
Meanwhile, diagnosing the state of perfusion of the cardiac chambers or the myocardium on the basis of a contrast echo image of the heart depends on the skill of a diagnostician, and an unskilled diagnostician encounters difficulty in correctly diagnosing such perfusion in a simple manner. This is because image processing has not conventionally been performed objectively, and quantitative analysis of the contrast echo image has not been carried out.